1. Field of the Invention
The present invention concerns an x-ray computed tomography apparatus of the type having a stationary device for generation of x-ray radiation from an x-ray focus that moves around the examination volume on a target that at least partially encloses an examination volume of the apparatus in one plane, wherein an x-ray beam is directed from the focus through the examination volume onto respective, temporarily opposite detector elements of a stationary x-ray detector that at least partially surrounds the examination volume, and wherein one or more shaping elements that influence one or more beam parameters of the x-ray beam are arranged between the target and the detector elements.
2. Description of the Prior Art
Computed tomography systems are used in medical imaging to acquire images of the inside of the body of a patient. A computed tomography apparatus includes, among other things, a device for generating x-ray radiation, an x-ray detector and a patient positioning table with which the examination subject can be moved in the examination volume along a system axis (the z-axis) during the examination. The device for generating x-ray radiation emits an x-ray beam that emanates from an x-ray focus rotating around the examination volume. In examinations, the x-ray beam, which expands in a fan shape perpendicular to the system axis in a slice plane of the examination volume (X-Y plane), penetrates a slice of the examination subject (for example a body slice of a patient) and strikes the detector elements of the x-ray detector opposite the x-ray focus. The angle at which the x-ray beam penetrates the body slice of the patient and, if applicable, the position of the patient positioning table normally continuously vary during the image acquisition with the computed tomography apparatus.
The intensity of the x-rays of the x-ray beam that strikes the x-ray detectors after penetrating the patient is dependent on the attenuation of the x-rays by the patient. Dependent on the intensity of the received x-ray radiation, each detector element of a detector row of the x-ray detector generates a voltage signal that corresponds to a measurement of the global transparency of the body for x-rays from the x-ray tube to the corresponding detector element. A set of voltage signals of the detector elements of a detector row, which represent attenuation data and were acquired for a specific position of the x-ray source relative to the patient, is designated as a projection. A set of projections acquired at various positions during the movement of the x-ray focus around the patient is designated as a scan. The computed tomography apparatus acquires many projections at various positions of the x-ray focus relative to the body of the patient in order to reconstruct an image that corresponds to a two-dimensional slice image of the body of the patient or a three-dimensional image. For acquiring a number of slice images or acquiring a three-dimensional image, a volume scan is implemented that encompasses a number of rotations of the x-ray focus around the examination volume given a feed movement of the patient table in the Z-direction. The prevalent method for reconstruction of a slice image or three-dimensional image from acquired attenuation data is known as filtered back-projection. The image reconstruction is normally implemented with an image computer that obtains the measurement data from the detector elements and further processes the data.
In a computed tomography apparatus of the third generation, the rotating x-ray focus is generated by an x-ray tube that, like the x-ray detector, is attached on a rotary frame that can be rotated around the examination volume. The rotation speed of the rotary frame has been increased in recent years in order to achieve faster scan speeds in the image acquisition. Even higher scan speeds, however, are required for new applications of computed tomography such as, for example, examination of the heart or blood circulation in vessels. In the meantime, for reasons of mechanical stability and safety in computed tomography systems of the third generation, a limit has been reached that no longer allows a distinct increase of the rotation speed of the rotary frame due to the masses that must be moved and the high acceleration forces resulting therefrom.
In a computed tomography apparatus of the fourth generation, the x-ray detector is arranged as a stationary ring around the examination volume such that only the x-ray tube must still be moved with the rotary frame. However, here as well significant forces that limit the maximum rotation speed act on the x-ray tube given a further increase of the rotation speed of the rotary frame.
To avoid this problem, in the meantime computed tomography systems of the fifth generation have become known in which both the device for generating x-ray radiation and the x-ray detector are stationary. In these computed tomography systems a target is used that at least partially encloses the examination volume of the apparatus in one plane. An x-ray focus moving around the examination volume is generated on the target, from which the x-ray radiation emanates. These computed tomography systems thus operate entirely without a mechanically-moving x-ray tube. The target extends either completely around the examination volume or at least over an angle of more than 180° around the examination volume. In the same manner, the x-ray detector encloses the examination volume either completely or over an angle of at least 180° and is arranged such that an x-ray beam emanating from the x-ray focus strikes (through the examination volume) on respective, momentarily opposite detector elements of the stationary x-ray focus.
For example, U.S. Pat. Nos. 4,158,142 and 4,352,021 disclose computed tomography systems of the fifth generation in which the target and the x-ray detector each completely surround the examination volume or surround it through an angle of 210°. To generate the x-ray focus, an electron beam is generated with an electron gun and the electron beam is directed over the target by suitable deflection. Such computed tomography systems can operate entirely without mechanically-rotating parts.
In another known embodiment of a computed tomography apparatus of the fifth generation as described in DE 40 15 105 C3, a target entirely enclosing the examination volume is used with a coaxial ring of electron sources is arranged close thereto. An x-ray focus rotating around the examination volume can likewise be generated by individual activation of the electron sources of the electron source ring.
U.S. Pat. No. 4,606,061 discloses a further embodiment of a computed tomography apparatus of the fifth generation in which an x-ray focus moving around the examination volume is generated on a target entirely enclosing the examination volume. An electron source ring coaxial to the target is provided that is activated for electron emission by a laser beam striking on its surface.
Due to the absence of a rotating x-ray tube, the aforementioned embodiments of computed tomography systems of the fifth generation achieve significantly higher scan speeds, but suffer from a reduced image quality and dose efficiency relative to computed tomography systems of the third generation. This is primarily a result of the limited possibilities for influencing beam parameters of the x-ray beam. In computed tomography systems of the third generation, a component known as a phi collimator is used that limits the aperture angle of the x-ray beam in the slice plane (X-Y plane) to the required FoV (Field of View). Unnecessary x-ray propagation outside of the region of interest, which leads to an increased scatter radiation and thus to a reduced image contrast and an increased radiation dose for the patient, is thereby avoided. In computed tomography systems of the fifth generation, no phi collimators are used due to the different technique for generation of the rotating x-ray focus. The same applies for filters for influencing the beam profile of the x-ray beam, for example a filter known as a bowtie filter that is arranged in front of the x-ray tube in computed tomography systems of the third generation. Such filters improve the dose efficiency by 15 to 20%, reduce the radiation dose for the patient, and prevent an over-exposure of the detector elements.
To improve the signal-to-noise ratio, scattered-ray grids to reduce the scatter radiation striking on the detectors are normally mounted on the incoming side of the x-ray detector. For an optimal suppression of the scatter radiation, the lamellae of such a scattered-ray grid should be aligned to the x-ray focus. In the known computed tomography systems of the fifth generation with a stationary scattered-ray grid, this cannot be realized with justifiable expense. The lamellae of the scattered-ray grid are therefore not aligned to the x-ray focus in the previously-described embodiments. In the known computed tomography systems of the fifth generation, the limitation of the x-ray beam in the Z-direction is achieved by a Z-collimator that is composed of two parallel, stationary rings that surround the examination volume within the target and detector ring, the rings therebetween defining the expansion of the x-ray beam in the Z-direction. Dependent on the mutual offset of the target and the x-ray detector in the Z-direction, this leads to a distortion of the ideally rectangular cross-section of the x-ray beam on the detector elements given the use of a multi-line detector, in particular to a banana-shaped or barrel-shaped distortion. FIG. 1 shows an example for a banana-shaped distortion of the cross-section 1 of the x-ray beam on an eight-line x-ray detector. The outer left detector lines 2 are not exposed in the center, or at least are less exposed in the center, while the outer right detector lines 2 are exposed only in the center. This banana-shaped beam profile on the detector can be avoided only when the Z-collimator is opened further, but this reduces the dose efficiency. If the Z-collimator is not opened as far, the outer detector channels are unusable since they detect no x-ray radiation or only very little x-ray radiation. This reduces the detector efficiency and the image quality and leads to increased partial volume effects.